This application claims the priority of German application 198 37 179.9, filed Aug. 17, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a transport belt for transporting a fiber strand to be condensed over a suction slit of a condensing zone, said belt having a perforation for a suction air stream which sucks the fiber strand.
In U.S. Pat. No. 5,600,872 a transport belt of this type is described, which is designed like a drafting apron, but made of a material which has a greater elastisicity than is usual in the case of drafting aprons. The transport belt comprises centrical holes arranged in travel direction, through which holes the suction air stream enters. The size of the perforations determines to what degree the fiber strand is bundled transversely to the transport direction in the condensing zone. The transport belt is guided during operation over a suction slit, which extends in transport direction and which is essentially wider than the perforation.
The condensing of an already drafted, yet still spinning twist-free fiber strand serves the purpose of rolling outwardly projecting edge fibers around the core strand, so that a better material utilization is permitted and that the fiber strand is less hairy before being imparted a spinning twist. This results in a smoother and more tear resistant yarn.
It has been shown that it is not favorable when the clearance of the perforation holes alone determines the degree of condensing. The diameter of the holes would have to be so large that the air entering though the perforations would become inhomogenous.
It is an object of the present invention to make the condensing effect not exclusively dependant on the clearance of the perforation, but rather to chose a perforation with which a homogenous as possible suction air stream can be achieved.
This object has been achieved in accordance with the present invention in that the transport belt comprises a non-perforated area which permits a friction drive, and an effective area containing the perforation, which effective area is wider than the width of the suction slit.
In contrast to prior art, the clearance of the perforation holes no longer determine the condensing effect, but rather the suction slit located under the transport belt. The width of the suction slit is somewhat wider than the width of the condensed fiber strand. The effective width of the perforation is in contrast significantly wider, namely so wide that the suction slit can, if required, be arranged under the perforation slightly transversely to the transport direction, in order that the fiber strand to be condensed is imparted an additional, slight false twist. In the case of such an embodiment the perforation can be so close-meshed that a completely homogenous suction air stream arises.
The transport belt can consist of a flexible apron looped around a drive roller, which apron comprises a plurality of centrical rows of holes. A thin steel belt of, for example, 0.4 mm thickness can be provided, which comprises a centrical perforation produced by means of etching. Particularly advantageous is, however, a skeleton type supporting structure for a transport belt, on which only a very thin perforated tape is applied in the central area. This centrical placed perforated tape can consist of a particularly thin and close-meshed woven or knitted fabric.
In a variation of the embodiment according to the present invention, the perforated tape is welded or adhered to the skeleton-like supporting structure. Alternatively, the perforated tape can be applied to the skeleton belt in an interchangeable way.
It is important that the perforation is as closely meshed as possible, while on the other hand the perforated area should be significantly wider than the fiber strand to be condensed. The perforation serves only for the generating of a suction air stream, which effects the pneumatic condensing. The degree of condensing is however determined by the suction slit located under the transport belt.